Here is some food for thought followed by a list of ideas that I have personally tried and found successful:
1) Plan way ahead of summer. You do not want to wait until summer to figure out what is offered in your area.
2) Think outside the box...or even your city. It is worth a thirty minute drive or more for your child to receive something you don't have locally.
3) Ask your friends or anyone you know for ideas.
4) Try to find out what your child is interested in and is good at by exposing your child to a variety of things. You might be surprised.
5) Try to be flexible. Do not overfill the calendar.
These are specific ideas broken down by age/maturity level:
Preschool: If you are a parent of a preschool child then consider continuing preschool through their summer program if it is offered. Although summers tend to be less structured, they still have consistency and that is key! Get together with other parents and try to do something weekly such as meeting at the park, pool, or going to fast food indoor playgrounds. Take turns meeting at different homes to provide social opportunities for you and your child. This is a fun age. Remember that they get excited about the smallest things as long as YOU are excited! Tell your child that you are going on a scavenger hunt and print off a list of items that you two can find while on a nature walk or a hike. Take notice of the things you see along the way and talk about them. Another idea is doing a backyard "safari". Get magnifying glasses and see how many creatures and wildlife you can find in your own backyard! Pinterest has some awesome ideas. I saw one where you get a large plastic tub and fill it with toys and then with water. Freeze it. Then take it outside and do an archeological "dig" to get to the toys inside the ice! Kids this age just LOVE sidewalk chalk.
Elementary: Kids are now a little more self aware. It might be harder to keep them occupied. You need to get some teaching resources to maintain their level in reading, math, and writing. Try to give them learning opportunities. Check your local library for Summer Reading programs or other special programs they may offer. Visit some historical sites or museums in your area. Take a tour at a local factory or other company that offers them. Invite friends over for a lunch/ dinner and a movie. Make cupcakes or pizzas with your child. Let your child do the measuring of ingredients by following a recipe. Play their favorite board game or just design your own. Science experiments using household items are a huge hit! Google for more ideas.
Teens: This is a challenging age. Going to the pool saved me during these years. However, on a rainy day you are stuck. If your child is a reader then you are set! Since they are a little more independent this age can also be so much fun. Whatever you do, do NOT call a get-together with a friend a "play date"! Ever! Go to a movie. Take advantage of Summer matinee deals around your area. Go bowling. Plan outdoor adventures such as a trip to the lake, canoeing, camping (when weather is mild). This doesn't sound like fun, but give them chores with rewards. This will save YOU in the long run. Plus, accountability is key to rewards and fun.
Hopefully this gave you some ideas and food for thought. Every child is different and so is every parent. There is no right or wrong. There is only what works for you. If it works, then it is right.